[Technical Field]
The present disclosure relates to fixing devices switching a pressure contact state of a heating structure and a pressure structure, image forming devices including such fixing devices, and pressure contact state switching devices used in such fixing devices, image forming devices, or the like.
[Description of the Related Art]
In electrophotographic image forming devices, an electrostatic latent image is formed by exposing and scanning a surface of a photoreceptor by light on the basis of image data of a document, supplying toner to the electrostatic latent image to form a toner image, transferring the toner image onto a recording sheet, and heat-fixing the toner image in a fixing device.
In a typical fixing device, a recording sheet is passed through a nip formed between a heated fixing roller (fixing rotational body) and a pressure roller (pressure rotational body) that is in pressure contact with the fixing roller and the recording sheet is conveyed forward while the toner image thereon is heat-fixed. When the recording sheet is jammed at the nip, the pressure contact of the pressure roller and the heating roller needs to be released by moving the pressure roller away from the heating roller in order to deal with the sheet jam.
Further, when special paper such as an envelope is passed through the nip, the special paper may wrinkle or the like under a strong pressure contact force. Accordingly, the pressure contact force when special paper is passed through the nip is beneficially smaller than a pressure contact force when plain paper is passed through (a contact state that is used when fixing a toner image on plain paper is hereinafter referred to as “full pressure contact state”, and a contact state that is used when fixing a toner image on special paper such as an envelope with use of a pressure contact force smaller than in the full pressure contact state is hereinafter referred to as “light pressure contact state”).
Thus, in recent years, a fixing device including a pressure contact state switching mechanism that is configured to switch a state of the fixing rotational body and the pressure rotational body (pressure contact state) between three states, namely the full pressure contact state, the light pressure contact state, and a release state, has been proposed (for example, Japanese Patent Application Publication No. 2017-009792 (hereinafter referred to as JP2017-009792)).
FIG. 12A, FIG. 12B, and FIG. 13 are schematic diagrams of a structure of a pressure contact state switching mechanism 50 in a fixing device pertaining to JP2017-009792.
In FIG. 12A, a heating roller 96 is supported by a main frame that is not illustrated, a pressure roller 97 is supported by a sub frame 60 through a shaft 27, and the sub frame 60 is supported at a lower end 51 of the sub frame 60 by the main frame through a swing shaft 28 to be swingable about the swing shaft 28. Due to this, by swinging the sub frame 60 clockwise, the heating roller 96 and the pressure roller 97 come to be in pressure contact.
A swing arm 52 is supported by the main frame through a swing shaft 56 to be swingable about the swing shaft 56, and an operation lever 62 is supported by the main frame through a swing shaft 65 to be swingable about the swing shaft 65. A pin 64 stands at an upper portion of the operation lever 62 towards a back of the fixing device and engages with a slot 63 of the swing arm 52. Due to this, a crank mechanism in which the swing arm 52 swings upwards and downwards about the swing shaft 56 in accordance with rotation of the operation lever 62 is formed.
A tension spring 53 for full pressure contact stretches between a spring hooking portion 61 at a right end of the swing arm 52 and a spring hooking portion 57 at an upper end of the sub frame 60. In the full pressure contact state in FIG. 12A, the tension spring 53 biases the sub frame 60 to rotate clockwise.
Further, at a back of the sub frame 60 relative to the front of the fixing device, a light pressure contact arm 54 overlapped with the sub frame 60 is supported by the main frame through the swing shaft 28 to be swingable about the swing shaft 28.
Between an upper portion of the light pressure contact arm 54 and a spring receiving portion 58 of the sub frame 60 is a compression spring 59 provided for light pressure contact and having a smaller spring constant than the tension spring 53.
In the state in FIG. 12A, the tension spring 53 for full pressure contact is extended and a restoring force of the tension spring 53 biases the sub frame 60 to the right, and therefore the pressure contact state is switched to the full pressure contact state. Here, a base end 55 of the operation lever 62 is not in contact with the light pressure contact arm 54, and therefore the compression spring 59 for light pressure contact has is at its natural length and does not bias the sub frame 60.
When the operation lever 62 is greatly rotated clockwise as in FIG. 12B, a left side of the swing arm 52 inclines downwards due to a crank function caused by engagement of the pin 64 of the operation lever 62 and the slot 63 of the swing arm 52. Thus, the spring hooking portion 61 of the swing arm 52 approaches the spring hooking portion 57 of the sub frame 60, and the tension spring 53 returns to its natural length and no longer biases the sub frame 60.
Further, a distance between the light pressure contact arm 54 and the spring receiving portion 58 does not change, and therefore the compression spring 59 remains at the natural length and does not bias the sub frame 60. Accordingly, the heating roller 96 and the pressure roller 97 are switched to a state in which pressure contact between the heating roller 96 and the pressure roller 97 is released (separated state).
When the operation lever 62 is caused to further rotate clockwise as in FIG. 13, while the swing arm 52 inclines further due to the crank function, a protrusion 66 of the operation lever 62 abuts against the upper portion of the light pressure contact arm 54 and causes the light pressure contact arm 54 to incline to the right. Accordingly, the tension spring 53 for full pressure contact remains at the natural length, the upper portion of the light pressure contact arm 54 approaches the spring receiving portion 58 of the sub frame 60, and therefore the compression spring 59 exerts an elastic force (restoring force) that biases the sub frame 60 to swing to the right (clockwise). Due to this, the heating roller 96 and the pressure roller 97 are switched to the light pressure contact state.
Switching from the light pressure contact state to the full pressure contact state is achieved by performing reverse operations of the above.
In the structure of JP2017-009792 above, a user has to open a door for maintenance of the image forming device and operate the operation lever 62 in order to switch the pressure contact state. Typically, when the door for maintenance is opened, supply of electric power to a driver including the fixing device is temporarily stopped for security. The image forming device is booted when the door is closed. However, a certain time period is required before image forming is performed, and therefore a forthcoming image forming job cannot be performed promptly.
Thus, in recent years, a demand for a structure in which the pressure contact state is switched automatically with use of a drive motor or the like is increasing.
However, even when the operation lever 62 described above is swung automatically by using a drive motor or the like, the fixing device of JP2017-009792 above inevitably switches the pressure contact state to the release state when switching the pressure contact state from the full pressure contact state to the light pressure contact state or from the light pressure contact state to the full pressure contact state as described above. Switching to the release state requires a certain time period, and a time period required before the first page of the forthcoming image forming job is output (Fcot: first copy time) becomes longer accordingly.